Effectiveness of whooping cough vaccine is studied

Scientists and public health officials are studying a Vermont whooping cough epidemic to determine if the current vaccine is less effective against a new strain sweeping the country.

For several years, scientists have noted a dramatic resurgence of the disease. In 2012, pertussis cases were up in 48 states, and Minnesota was among a handful of states, including Colorado, Washington, Wisconsin and Vermont, where outbreaks were particularly bad.

The Centers for Disease Control and Prevention and the Vermont Health Department are examining the records of 854 people, ages 4 to 19, with confirmed or probable cases of pertussis from late 2011 into early 2013.

They are also examining a control group of 2,500 people from the same age group and the same area who did not get sick. They want to compare their vaccination history against those who did contract whooping cough.

"Since 2010 in the U.S., we have documented some genetic changes in the circulating strains of pertussis," said Patsy Kelso, an epidemiologist from the Vermont Health Department. "We don't know how much or whether the changes in pertussis are impacting vaccine effectiveness."

Whooping cough, also known as pertussis, is a highly contagious disease that can strike people of any age but is most dangerous to children. Its name comes from the sound children make as they gasp for breath. In rare cases, pertussis can be fatal.

Vermont saw 18 cases of whooping cough in 2010, 94 cases in 2011, 645 in 2012 and 114 in 2013.

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So far this year there have been 29 cases, Kelso said.

The pertussis vaccine is known to lose its effectiveness over time. Nevertheless, Kelso emphasized that children should still get the five-shot series of pertussis vaccine between 2 months and before starting school. Public health officials are now recommending that teenagers, women in the third trimester of each pregnancy and adults get pertussis boosters.

"We still strongly recommend vaccine," Kelso said. "Vaccinated people can get pertussis, but there is good data from some recent studies that people who have been vaccinated have less severe disease."

The new strain doesn't appear to make people sicker than the earlier strain and isn't more contagious. But the CDC-led study is an attempt to determine if the vaccine used now is less effective against the new strain.

"We rushed to do this study in Vermont after we saw this change," CDC epidemiologist Stacey Martin said.

In 2010, less than 10 percent of whooping cough cases in the U.S. involved the new strain, while in 2013, more than 90 percent of the cases were from the new strain.

"To have that happen so quickly across the entire U.S. is something that we needed to better understand," Martin said.

After the information is gathered, the CDC will analyze the data. The results are expected this fall and could have widespread implications, Kelso said.